(a) Field of the Invention
The present invention relates to a method of producing a cast coated paper. More particularly, the invention relates to a method of effectively obtaining a cast coated paper which is superior in gloss, printability, abrasion resistance and water resistance to conventional cast coated papers.
(b) Description of the Prior Art
Conventional methods of producing a printing paper having a high gloss called a cast coated paper include a wet casting method, a rewet casting method and a gel-casting method. Said wet casting method comprises a base paper being coated with a cast coating composition, the main components of which are pigments and adhesives, then said base paper being pressed against the highly polished surface of a heated drum when the coating layer is still wet so that the paper is dried and glossed. Said rewet casting method comprises a coating composition in a wet state being dried once, rewetted and plasticized by means of a rewet liquid, and then pressed against the highly polished surface of a heated drum. Said gel-casting method comprises a coating composition in a wet state being gelled and pressed against the highly polished surface of a heated drum so that the paper is dried and glossed.
All of said casting methods are the same in that the coating layer in a wet, plasticized state is pressed against the highly polished surface of a heated drum, dried thereby and removed from said heated drum so that said coating layer copies the highly polished surface of said drum. Cast coated papers thus obtained have a high gloss and surface smoothness as compared with conventional super-calendered coated papers, and ensure a superior printing effect. Therefore, said cast coated papers are used particularly as high-grade printing papers and materials of high-grade paper ware, etc.
With the improvements of the grade of printed matters, book covers, paper ware, etc., it has been demanded that the gloss should be further improved over the conventional cast papers and the water resistance and abrasion resistance should be made higher. Methods used at present as means for satisfying such quality demands include a method of making a conventional coated paper, cast coated paper, etc. into a varnished paper or a press coat by coating said coated paper, cast coated paper, etc. with a transparent resin by a printing means and a method of making said papers into a paper laminated with a plastic film of polyethylene, vinyl chloride, etc. In all of these methods, said conventional coated paper and cast coated paper are subjected to secondary treatment.
Since it is impossible to print said varnished paper, press coat or laminated paper directly with a printing ink, these papers are varnished or laminated after being printed. It is inconvenient to do so. Particularly, said laminated paper widely used is very difficult to recycle because the laminated film thereof hinders defiberization and furthermore there is an excessive burden in the recycling process. Thus the laminated paper has many disadvantages in terms of the environmental problem, etc. over the conventional coated paper and cast coated paper.
It is an object of the present invention to obtain a cast coated paper good enough to be used in place of said varnished paper, press coat and laminated paper.
It is another object of the invention to obtain a cast coated paper having a gloss, ink gloss, abrasion resistance, water resistance etc. much better than those of said varnished paper.
Discussion will now be made as to what improvements can be made on the present technical level in an attempt to obtain a cast coated paper having high qualities comparable to those of said varnished paper, etc.
The composition of a conventional cast coating layer comprises a coating pigment and an adhesive generally used in the field of coated papers for printing, said pigment normally being used in an amount of 100 parts by weight per 5 to 50 parts by weight of said adhesive. Since the main component of said coating layer is the coating pigment, the cast coated paper obtained is superior in the absorption and retention of printing ink but much inferior in gloss, ink gloss, abrasion resistance and water resistance to said varnished paper, press coat and laminated paper.
It is possible to improve the gloss and ink gloss of a cast coated paper to some extent by increasing the amount of the adhesive in the cast coating composition. If the amount of the adhesive is increased, however, the porosity of the coating layer is lost, and the vapor permeability of said layer and the releasability from the polished drum is reduced. This will make the production speed mu(:h lower and extremely deteriorate the high printability, particularly ink set and ink drying, which is a characteristic feature of the cast coated paper.
It may seem possible to add a known release agent to the coating composition or increase the amount of the pigment in order to improve said releasability. However, the addition of the release agent alone cannot improve the releasability to a satisfactory level. If the pigment is added to such an extent as that said releasability is improved, the gloss will be reduced.
Also, it may seem possible to add a lubricant such as a polyethylene wax and a natural wax to the coating composition in order to improve the abrasion resistance of the cast coated paper. However, to obtain abrasion resistance comparable to that of said varnished paper, press coat or laminated paper, it is necessary to add said lubricant in large quantities and as a result printability such as ink set and surface strength may by reduced.
The water resistance of the cast coated paper can be improved by adding a water resisting agent which has been used in the field of paper coating. However, it is difficult to obtain water resistance equal to that of said varnished paper, press coat or laminated paper.
As apparent from the above, it is very difficult to find in the present technical level a means for satisfying all of said qualities, printability and releasability of the cast coated paper . In other words, even if an improvement is made on the basis of the conventional method of producing the cast coated paper, it is very difficult to obtain gloss, ink gloss, abrasion resistance and water resistance comparable to those of said varnished paper, press coat or laminated paper.
A case is known in which an attempt was made to improve the qualities of a cast coated paper by means of a rewetting method by improving a rewet liquid used in a reset casting method. For example, Japanese Patent Publication No. Sho 48-38005 discloses cast finishing by a rewet casting method by means of a rewet liquid containing a film forming substance in an amount of 0.1 to 20% immediately before a coating layer comprising a pigment and an adhesive is pressed against the highly polished surface of a heated drum. In this method, said coating layer comprising a pigment and an adhesive is in charge of printability such as the absorption and retention of ink, and an attempt is made to improve gloss by forming a thin layer of said film forming substance on the surface of said coating layer. However, the film forming substance can be added only to such an extent that the porosity of the coating layer is not lost and it is impossible to obtain sufficient gloss.
It is possible to improve gloss by increasing the amount of said film forming substance within said rewet liquid. Since in this case a thick layer of said film forming substance is formed on the surface of the pigment in the coating layer, the absorption of ink by the pigment is hindered and ink set is reduced. Since vapor permeability is reduced, releasability is also deteriorated. To obtain satisfactory printability and releasability on these conditions, the film forming substance itself must be excellent in the absorption, retention, etc. of ink and releasability. However, the film forming substance shown in said publication does not satisfy these requirements.